1. Field of the Invention
The present invention relates to a modification process of a synthetic quartz powder, wherein it is possible to make a quartz glass having few carbon content and hydroxyl group content and hardly having an inner bubble content at the time of fusing to be vitrified at high temperature, and also relates to a glass product using said quartz powder.
2. Discussion of the Background
It has been known that a process, in which a gel made by hydrolyzing a metal alkoxide etc. is dried and baked to produce the synthetic quartz powder, i.e., a sol-gel method. Conventionally, it has been known that the synthetic quartz powder made by said producing process is amorphous and has high purity having fewer metal impurities than the natural quartz powder. However, it has been also known that there are a residual carbon or a residual hydroxyl group from an alkoxy group in said synthetic quartz powder. Therefore, when the quartz glass crucible is produced by using the synthetic quartz powder made by the sol-gel method as a raw material, said residual carbon etc. becomes the cause of the inner bubbles.
Then, regarding the synthetic quarts powder produced by the sol-gel method, many trials for reducing the residual carbon have been carried out. For example, the following processes etc. have been already known. The process, in which the dried gel powder is heated to remove the residual carbon by burning at the temperature of less than 600° C., in which the pores in the powder are not closed, and after this decarbonization, said heated powder is baked to be vitrified at the high temperature being about from 1000° C. to 1300° C., in which the pores are closed, to produce the amorphous synthetic quartz powder, i.e., shown in Japanese Patent Laid Open No. H09-86916 or No. H10-287416, etc. The process, in which the silica powder produced by the sol-gel method is heated at about 500° C. in air to burn organic components, and is vitrified by heating to fuse at more high temperature to remove the OH group. The process, in which the dried gel powder is baked in two steps at a dried atmosphere or a reduced atmosphere at the time of making the synthetic quartz powder to bake the dried gel powder, and then, the synthetic quartz powder having the predetermined high density is produced i.e., shown in Japanese Patent Registration No. 2530225. However, it is impossible to reduce sharply the residual carbon in the quartz powder by these conventional producing processes, so that it is substantially difficult to produce the glass products hardly having inner bubbles.
On the other hand, it has been known that the process, in which the quartz powder produced by the sol-gel method is carried out by the heat treatment at the temperature being from 800° C. to 1300° C. in hydrogen atmosphere, to make the quartz powder which does not indicate a spectral peak of O═C═O bonding, and the quartz crucible is produced by using said quartz powder as the raw material, i.e., shown in Japanese Patent Laid Open No. 2001-261353. However, in this process, there is a problem that the concentration of OH basis and the amount of an adsorbed water in the quartz powder become high by the hydrogen gas, so that the inner bubbles are increased by the adsorbed water etc., when the quartz powder is vitrified. Moreover, it is dangerous that the hydrogen gas is introduced at the high temperature. Furthermore, it has been also known that the process other than these processes, in which after a fumed-silica produced by the sol-gel method is carried out the heat treatment, said fumed-silica is baked in vacuum, hydrogen, or helium atmosphere at high temperature to make the quartz glass powder, i.e., shown in Japanese Patent Laid Open No. 2001-89168.
Although all of the above-mentioned conventional modification processes are aimed at the removing of the residual carbon, the cause of the bubble aroused at the time of vitrifying the quartz powder is not limited to the residual carbon. The air components adsorbed to the quartz powder after the heat treatment also become the important cause of the inner bubble. Regarding the present invention, it is possible to make the quartz glass crucible which has no bubbles substantially by carrying out the heat treatment and also preventing the adsorption of the air components after said heat treatment. In addition, the above-mentioned problems in the conventional modification processes for producing the quartz powder are also solved.